It is now sufficiently clear that mineralocorticoids are potentially important participants in energy regulation to justify the launching of a sustained research effort. Focusing on aldosterone, the major mineralocorticoid in rat and man, we propose to begin defining the features of the phenomenon that appear most fruitful from the present perspective. Among these, and perhaps most pivotal, is an analysis of carcass composition following hormone treatment. This will indicate whether the steroids are promoting fat deposition, as present evidence indicates, or if lean body mass or water are involved. Along with this, we address the question of magnitude of effect--whether aldosterone infused over an extended duration results in progressive or limited effects, and if sex and age are interactive. The results of these studies will begin to outline the importance of mineralocorticoids for obesity research: whether they have sufficient impact across age and sex, and are of a magnitude great enough to bear any clinical significance, or if the phenomenon under study is best pursued as a problem of basic endocrinology. Ranking in importance with the effects of aldosterone on the body are behavioral considerations. Preliminary findings indicate that aldosterone alters fat metabolism, promoting deposition and inhibiting mobilization. This may have the consequence of creating a specific preference for high fat diets. If this is the case, then a thorough analysis is indicated, for any agent capable of shifting preferences toward increased dietary fat consumption constitutes a significant health risk. Current nutritional and epidemiological research has established reliable links between dietary fat and obesity, cancer, hypertension, and immunological suppression. We propose to search for any aldosterone-induced preference shifts in self-selection and forced-acceptance designs that can help determine if the shifts spring from pre- or postingestive changes.